Add Preconnect.

Part of #25.

Sources/Slipstream/Documentation.docc/Guides/SlipstreamForWebDevelopers.md

@@ -46,7 +46,7 @@ provided below is an organized table of W3C HTML tags and their equivalent Slips
 [`<link rel="noreferrer">`](https://html.spec.whatwg.org/multipage/links.html#link-type-noreferrer) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 
 [`<link rel="opener">`](https://html.spec.whatwg.org/multipage/links.html#link-type-opener) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 
 [`<link rel="pingback">`](https://html.spec.whatwg.org/multipage/links.html#link-type-pingback) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25)   
-[`<link rel="preconnect">`](https://html.spec.whatwg.org/multipage/links.html#link-type-preconnect) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 
+[`<link rel="preconnect">`](https://html.spec.whatwg.org/multipage/links.html#link-type-preconnect) | ``Preconnect`` 
 [`<link rel="prefetch">`](https://html.spec.whatwg.org/multipage/links.html#link-type-prefetch) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 
 [`<link rel="preload">`](https://html.spec.whatwg.org/multipage/links.html#link-type-preload) | ``Preload`` 
 [`<link rel="prev">`](https://html.spec.whatwg.org/multipage/links.html#link-type-prev) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 

Sources/Slipstream/Documentation.docc/W3C/W3CViews.md

@@ -15,6 +15,7 @@ The complete W3C HTML elements standard can be found [here](https://html.spec.wh
 - ``Head``
 - ``Title``
 - ``Preload``
+- ``Preconnect``
 - ``Stylesheet``
 - ``Meta``
 - ``Charset``

Sources/Slipstream/W3C/Elements/DocumentMetadata/Preconnect.swift

@@ -0,0 +1,41 @@
+import Foundation
+
+import SwiftSoup
+
+/// A view that indicates that preemptively initiating a connection to the
+/// origin of the specified resource is likely to be beneficial, as it is highly
+/// likely that the user will require resources located at that origin, and
+/// the user experience would be improved by preempting the latency
+/// costs associated with establishing the connection.
+///
+/// ```swift
+/// struct MySiteMetadata: View {
+///   var body: some View {
+///     Head {
+///       Preconnect(URL(string: "https://rsms.me/"))
+///     }
+///   }
+/// }
+/// ```
+///
+/// - SeeAlso: W3C [link](https://html.spec.whatwg.org/multipage/semantics.html#the-link-element) specification.
+/// - SeeAlso: W3C [preconnect](https://html.spec.whatwg.org/multipage/links.html#link-type-preconnect) specification.
+@available(iOS 17.0, macOS 14.0, *)
+public struct Preconnect: View {
+  /// Creates a Preconnect view.
+  public init(_ url: URL?) {
+    self.url = url
+  }
+
+  @_documentation(visibility: private)
+  public func render(_ container: Element, environment: EnvironmentValues) throws {
+    guard let url else {
+      return
+    }
+    let element = try container.appendElement("link")
+    try element.attr("rel", "preconnect")
+    try element.attr("href", url.absoluteString)
+  }
+
+  private let url: URL?
+}

Tests/SlipstreamTests/W3C/PreconnectTests.swift

@@ -0,0 +1,14 @@
+import Foundation
+import Testing
+
+import Slipstream
+
+struct PreconnectTests {
+  @Test func nilURL() throws {
+    try #expect(renderHTML(Preconnect(nil)) == "")
+  }
+
+  @Test func validURL() throws {
+    try #expect(renderHTML(Preconnect(URL(string: "https://rsms.me/"))) == #"<link rel="preconnect" href="https://rsms.me/" />"#)
+  }
+}